Science fish eggs
Prof. Hiramatsu's Laboratory of Aquaculture Biology, Division of Marine Life Science
토픽 개요
Science fish eggs × SDGs
As one of the 17 Sustainable Development Goals (SDGs) set forth by the United Nations, “conservation and sustainable use of ocean and marine resources” is a global challenge. In recent years, due to the decrease of natural fishery product resources because of the changes in the marine environment and the increase of global demand for fishery products, efficient and sustainable development of aquaculture technology for increasing fishery products is demanded.
In our lab, under the keyword "Science fish eggs", we are developing efficient fish resource survey techniques and biotechnology using fish, in addition to seed production technology development and breeding tests using advanced technology based on physiological basic research on fish egg formation. Through the research, we hope to contribute to the SDGs 2 ZERO HUNGER and 14 LIFE BELOW WATER.
The United Nations has designated the decade starting in 2021 as the "Decade of Ocean Science," with the aim of contributing to SDGs. Ocean science, as defined by the UN, includes the field of fisheries.
Summary
As one of the 17 Sustainable Development Goals (SDGs) set forth by the United Nations, “conservation and sustainable use of ocean and marine resources” is a global challenge. In recent years, due to the decrease of natural fishery product resources because of the changes in the marine environment and the increase of global demand for fishery products, efficient and sustainable development of aquaculture technology for increasing fishery products is demanded. In our lab, under the keyword "Science fish eggs", we are developing efficient fish resource survey techniques and biotechnology using fish, in addition to seed production technology development and breeding tests using advanced technology based on physiological basic research on fish egg formation.
Development of technology for efficient production of fish seedlings
It is extremely important to develop stable seedling production technologies when fish are farmed. However, there are many fishes that do not spawn just by putting them in a tank.
【Examples of eels and fox jacopever】
Seedling production is the process of making fry for farming. The first stage of aquaculture begins with collecting and growing natural fry or gathering parents for spawning, but this cannot be said to be sustainable. Once a fry grows to an adult fish and the eggs are harvested, the seedling production can be sustained by farmed fish alone without relying on natural resources. In the case of Japanese eel, fries are overfished and endangered because we cannot gather parent fish in the fields during the spawning season. It is necessary to make fish raised from fry spawning, but they do not naturally lay eggs in the aquaculture environment. In addition, some fish, such as the fox jacopever, a kind of a viviparous fish, requires a long time to mature, and male spermatozoids can hardly be produced in the aquaculture environment. It is believed that the cause is the failure to turn on the switch that produces the necessary hormones at each stage from sexual maturation to spawning.
Artificial maturation
When environmental and physiological factors are arranged, the “gonadotropin-releasing hormone” is produced from the hypothalamus, and the “gonadotropin” is produced from the pituitary gland. “Gonadotropin” acts on the cell layer that surrounds the oocyte (follicular cell layer), producing sex steroid hormone and maturation-inducing hormone, which promotes egg growth and egg maturation, leading to egg laying.
As shown above, various hormones are involved. If sexual maturation and spawning do not occur, hormones are artificially administered depending on the stage of maturation that is stopped. This is called artificial maturation.
Assesment of Sex and Maturity
When administering hormones, it is important to know whether they are male or female, what stage of maturity they are in, and whether the maturation has progressed after hormone administration. “Female sex hormones” released from the follicle cells and the “oogenesis-related protein” released from the liver can be used as markers for determining sex and maturity.
immunochemical measurement method
“Hormones” and “oogenesis-related protein” of fish can be detected and measured by blood tests. For example, the common female sex hormone in fish is a steroid hormone called estradiol 17β (E2), which is the same female sex hormone found in the blood of women. In this way, many steroid hormones are common to vertebrates, and commercially available kits can be used to measure the hormones. On the other hand, protein-related hormones such as gonadotropin and oogenesis-related proteins partially differ in structure depending on the species, and we have to develop detection and measurement methods for each species respectively. While there are various methods for detecting and measuring steroids and proteins in the blood, one of the cheaper and more common methods is an immunochemical measurement method using an antigen-antibody reaction.
Research Example (1)
The first step o developing an immunochemical detection and measurement method is to prepare a highly pure protein of interest as an "immune antigen." The process of increasing the purity of a target protein from a sample containing multiple proteins is called "purification."
Example of purification of oogenesis related protein:
The yolk protein precursor is called vitellogenin (vitellogenin: Vg). The figure on the left is a flowchart of the purification of three types of Vg from the blood of a mullet (Mugil cephalus) to which female hormones have been administered. For example, in the case of VgA, we first use one POROS HQ of the ion-exchange column, followed by a negative affinity column combining the hydroxyapatite column and VgB acid, and finally one Superose 6 of the gel filtration column, which is purified through a total of four-column chromatographies.
Example of the chromatogram of gel filtration column:
Gel filtration column is a method to separate based on the differences in the molecules of protein. The figure on the left is a VgA elution chromatogram with the vertical axis indicating the absorbance value and the horizontal axis indicating the elution fraction number. Generally speaking, if you get one symmetrical peak, we think it is highly purified.
Research Example (2)
When rabbits are injected with purified fish proteins, the rabbits recognize them as foreign substances and produce antibodies. This antibody binds to the injected protein but not to the others. A chromogenic/luminescent substrate is attached to the antibody, and the amount of binding is detected and measured as color or luminosity.
Immunochemical assay systems for yolk protein precursors:
The figure on the left shows the standard curve (luminance on the vertical axis and VtgAa concentration on the horizontal axis) and the serum dilution series (Male serum ○, serum containing VtgAa ●) of the chemoluminescence immunoassay system for the yolk protein precursor (synonymous with VtgAa = VgA) of mullet (Mugil cephalus). The higher the VtgAa, the higher the luminosity value, and antibodies don′t react with other contaminating seroproteins (male serum).
Example of measurement of yolk protein precursors:
The figure on the left shows the immunochemical measurement of three types of Vtg using mullet serum at various stages of sexual maturity. The vertical axis is Vtg concentration, and the horizontal axis is gonad weight per body weight (gonad body index: GSI). Early in sexual maturation, Vtg levels are low. When GSI exceeds 10% and Vtg levels tend to decrease as spawning approaches. Continuous monitoring of Vtg as a blood marker for maturity and egg maturation can predict the timing of spawning and hormone administrations.
Terminology
Hypothalamus-pituitary system: The hypothalamus is a part of the diencephalon and is located above the pituitary gland and functions as the center of the autonomic nervous system, as well as regulating pituitary gland function. Neurosecretory cells that secrete neurohormones are present in the hypothalamus, and the neurosecretory cell axons of teleosts enter the pituitary gland and regulate pituitary hormone secretion. In the pituitary gland, various protein hormones (such as growth hormone, prolactin, somatolactin, melanophore-stimulating hormone, thyroid-stimulating hormone, gonadotropin, and adrenocorticotropic hormone) are synthesized, released into the blood, and act on target organs (eg, gonadotropin → gonad) at the periphery.
Ovary, follicle, oocyte: The gonads of the female fish are called ovaries, and are often paired left and right, and are suspended from the wall of the back body cavity by the mesovarium. The inside of the ovary is composed of ovarious lamellas, where numerous follicles are present. Follicles consist of follicular tissue and oocytes. The oocyte undergoes ovulation through the egg growth and maturation phases to become an egg, which leads to fertilization with spermatozoa.
Sex steroid hormones: The gonads produce a variety of hormones, including sex steroid hormones. Sex steroid hormones are synthesized using cholesterol as precursors. Those produced in the ovaries are called female hormones (estrogens and progestins), and those produced in the testes are called male hormones (androgens). In a narrow sense, the estrogen is sometimes synonymous with the female hormone. A typical fish estrogen is estradiol 17β (abbreviated as E2), which acts on the female liver to promote the synthesis of vitellogenin, a yolk protein precursor.
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